Demodex is a genus of ecto-parasitic mites within the Arachnida class that live in or near hair follicles of mammals. Demodex mites have a commensal relationship with humans, and they asymptomatically parasitize the pilosebaceous follicles of humans. Thus, these mites are part of the normal microbiome in the sebaceous areas of the skin in humans (Grice E A et al., “The skin microbiome: potential for novel diagnostic and therapeutic approaches to cutaneous disease,” Semin Cutan Med Surg., 2014, 33(2):98-103). In healthy skin, the density of the mites is normally low, and this low density does not cause skin diseases. When the density of these mites increases, they induce facial skin conditions. Patients with an ocular Demodex infestation often complain of itching, burning, redness, crusting at the base of the lashes, blurry vision and dry eye.
Among the over 100 species of Demodex mites that have been identified, Demodex folliculorum and Demodex brevis are the two main species commonly found on humans (Lacey N et al., “Under the lash: Demodex mites in human diseases,” Biochem (Lond)., 2009 Aug. 1, 31(4):2-6). Most individuals become colonized during childhood and numbers proliferate in and around the pilosebaceous units at the age of puberty as levels of secreted sebum increase, and increasing in prevalence with age (Elston D M, “Demodex mites: facts and controversies”, Clin Dermatol, 2010, 28(5):502-504). This may be due to lipases produced by Demodex, allowing the mite to utilize sebum as a food source (Jimenez-Acosta F et al., “Demodex mites contain immunoreactive lipase”, Arch Dermatol, 1989, 125(10):1436-1437). Other nutritional sources include cellular debris and bacteria that reside in and around the pilosebaceous unit.
D. folliculorum has a comparatively long body and proliferates at the base of the lashes (cylindrical dandruff), causing anterior blepharitis. Specifically, D. folliculorum consume epithelial cells at the hair follicle causing lash distention, hypoplasia, and reactive hyperkeratinization, which is commonly observed as trichiasis and madarosis. D. brevis is shorter in size and burrows into the sebaceous and meibomian glands, causing posterior blepharitis. D. brevis obstructs meibomian gland openings, leading to insufficient tear lipid secretion, which can be an inflammatory trigger for dry eye.
Demodex mites also carry their own bacterial reservoirs that can contribute to ocular surface inflammation. The bacteria on the surface of the mite has been shown to compete with known staphylococcal species, producing a human host immune response leading to an inflammatory eyelid and periorbital epidermal to subepidermal reaction. If the disease is in a chronic and progressive phase, the inflammation may spread to the conjunctiva and cornea, potentially leading to infiltrative keratoconjunctivitis, nodular scar deposition and corneal neovascularization.